1. Field of the Invention
The present invention relates to a cannula for medical use and more particularly to a double lumen cannula for independently conducting fluids into and out of a venipuncture site through two separate passageways.
2. The Prior Art
Double lumen cannulae are well known in the art. The best known advantage offered by double lumen cannulae is the ability to separately inject fluids into a blood vessel and withdraw fluids from a blood vessel through a single venipuncture.
The most common double lumen cannulae consist of a single tube with a horizontal division of the tube which places the lumens of the cannulae in immediate juxtaposition. Unfortunately, however, the construction of such double lumen cannulae is expensive, time consuming and unreliable.
The least expensive and most reliable construction of the double lumen cannulae is in the form of concentric lumens disposed telescopically one within the other. Especially in using the cannulae for single needle dialysis, it is preferred to have one of the cannula lumen project substantially beyond the other. The problems with such telescopic construction are apparent, however, when it is observed that both cannula must be used to penetrate the skin during venipuncture. Unless there is a smooth contour between the interior and exterior cannula, the venipuncture is both difficult and painful. On the other hand, if the telescoping cannulae present an exteriorly smooth surface for venipuncture, there is insufficient passageway for fluid to flow easily between the lumen. Accordingly, until this present invention, the construction of a safe, effective and comparatively painless double lumen cannula having telescoping lumen has not been known.